A hologram is obtained by recording a wave front of object light as an interference fringe which is formed by interfering two kinds of lights having the same wavelength (object light and reference light) on a photosensitive material. When light having the same condition as that of the original reference light is irradiated to the hologram, a diffraction phenomenon due to the interference fringe occurs, and the same wave front as that of the original object light can be reproduced.
The hologram can be classified according to a recording form of the interference fringe.
Recently, the so-called hologram for recording the interference fringe according to a difference in refractive index in the interior of a recording layer has been applying for use such as in three-dimensional display, optical element and the like because of it's high diffraction efficiency and excellent wavelength selectivity.
As a photosensitive material for recording such a volume hologram, those compositions comprising silver halide or gelatin dichromate which have hitherto been used in the fields of art are popular. However, these compositions are not suitable for manufacturing the hologram industrially because wet development and complicated developing/fixing treatment are required. Further, there is a problem that the resulting image after recording disappears due to moisture absorption.
In order to overcome the above problem of conventional technique, U.S. Pat. Nos. 3,658,526 and 3,993,485 suggest preparing a volume hologram only by a simple dry treatment using a photopolymer. The hologram due to the photopolymer is estimated to proceed by a formation mechanism studied in "APPLIED OPTICS", B. L. Booth, Vol. 14, No. 3, PP 593-601 (1975); and W. J. Tomlinson, E. A. Chandross et al., Vol. 15, No. 2, PP 534-541 (1976). However, these techniques could not hardly compete with the above-described technique in view of refractive index modulation which is particularly important performance.
Improved techniques thereof are proposed in, for example, U.S. Pat. Nos. 4,942,102 and 4,942,112, Japanese Laid-Open Patent Publication No. 3-249685 and Japanese Patent Application No. 4-67021. In these techniques, a high refractive index radical polymerizable compound is used in combination with a non-reactive plasticizer, a radical polymerizable compound or a cationic polymerizable compound in order to improve refractive index modulation capability. In that case, a compound containing an atom (e.g. chlorine, bromine, sulfur, etc.) which contributes to high refractive index is used as the high refractive index radical polymerizable compound. In this case, compatibility of the high refractive index radical polymerizable compound with the non-reactive plasticizer or the cationic polymerizable compound is sometimes poor, and there is a problem such as insufficient dissolution of the composition, haze of the photosensitive material and the like. Further, when a compound free of an atom such as chlorine, bromine, sulfur, etc. which contributes to high refractive index is used for the high refractive index radical polymerizable compound, sufficient refractive modulation can not be obtained.
On the other hand, Japanese Laid-Open Patent Publication No. 5-32807 discloses an epoxy acrylate compound having a 9,9-diarylfluorene skeleton as the high refractive index radical polymerizable monomer containing no atom as described above. However, since epoxy acrylate compounds are solid at ambient temperature, the photosensitive material is hazy and, therefore, sufficient light transmission property cannot be obtained.